1. Field of the Invention
This invention relates to a nondestructive evaluation/inspection sensor and method for detection of corrosion in bared and painted metals and metallic alloys.
2. Background of the Related Art
Airline fleets operating for extended periods beyond their original design life-time depend on the ability to anticipate required adjustments in the inspection and maintenance activities to compensate for the aging aircraft. Aircraft metallic structures often suffer from corrosion during operation or storage, especially in humid, rainy, hot and seriously polluted areas. Corrosion may shorten the aircraft service life, increase maintenance costs, and cause structural failures. The development of inexpensive, practical, and user friendly nondestructive evaluation/inspection (NDE/I) techniques to detect corrosion with a high degree of accuracy, sensitivity and versatility is needed in the industry.
Aircraft metallic structures may suffer from corrosion even when they are covered with primers, coatings, and paint. Aircraft metallic structures can harbor deterioration that is un-detectable by visual inspection. Most of the NDE/I methods currently in use are based on detection principles which involve measurements of macroscopic changes in aircraft structures such as pits, crevices or cracks with sizable dimensions. These NDE/I techniques are not capable of detecting the early stages of corrosion initiation. Existing techniques also do not warn in advance of critical areas where possible severe corrosion could occur. In addition, existing NDE/I techniques for the detection of hidden corrosion present other practical limitations such as the need for highly trained personnel, cost, time, and the use of complicated and even hazardous instrumentation. Thus, the development of an inexpensive, user friendly, fast, and portable novel NDE/I system for the detection of hidden corrosion in aircraft structural components is highly desirable. Additionally, the new NDE/I system should be able to detect hidden corrosion in early stages so that both structural damage can be prevented and maintenance procedures can be minimized. Methods to both detect corrosion initiation of metallic structures and assess the amount and severity of damage to the metallic structures are critical to avoid failure of the structures and allow for the implementation of a systematic, cost effective approach to repairing and/or replacing damaged structures.
Corrosion of metallic structures is an electrochemical process per se, and several electrochemical techniques have been used in the past to detect and monitor corrosion of metals. One popular electrochemical technique for the detection and monitoring of corrosion is electrochemical impedance spectroscopy (EIS). This technique applies a small AC voltage perturbation across the substrate under study and measures the perturbation in the current response of the substrate. The impedance of the substrate (defined as the ratio between the applied potential and the measured current) is determined as a function of the frequency of the signal. The use of non-steady-state techniques with low amplitude perturbation signals (e.g., EIS) has become very attractive in corrosion research and testing, because it provides additional information on the kinetics of the corrosion process without significantly affecting the substrate under investigation. The majority of these techniques however, employ the use of liquid electrolytes. Liquid electrolytes (e.g., NaCl, H2SO4) typically damage the substrate being tested because of the presence of aggressive ions.
Painting and coating with organic polymer materials is a widely used method to provide corrosion protection to metallic substrates. The protective coating not only impedes the ingress of corrosive materials, but also decreases the electrical transport between anodic and cathodic sites on the metal surface. However, paint films are generally not impervious to reactants such as water, water vapor, oxygen and ions (e.g., chloride). The degradation of painted metals with time is characterized by many individual partial processes acting together, such as UV degradation, the permeation of water, oxygen and ions through the paint film, loss of color and gloss, changes in film adhesion with both degradation and wet/dry stages, and chemical degradation of the paint film. When the paint film is left deteriorated, corrosion of a base metal covered by the paint film finally causes flaking of the paint film and occurrence of rust. A non-destructive method for detecting corrosion even under painted surfaces would be very useful.
One embodiment of the present invention provides an apparatus for detecting corrosion. The apparatus has a chamber with an open end portion, a solid electrolyte, such as an ion exchange membrane, coupled over the open end portion of the chamber, and an electrode in electrochemical communication with the solid electrolyte. Preferably, the apparatus is self-contained leak-proof unit. A biasing member may be disposed in the chamber for biasing the electrode into contact with the electrolyte, and ultimately into contact with the substrate being tested. The open end portion of the chamber can be generally angular, stepped, concave, convex, or flat so that it may be adapted to fit the geometry of the substrate being tested. Preferably at least a portion of the chamber is made of an electronically and ionically insulating material. Preferably, the solid electrolyte is electronically insulating.
Preferably, an inert insulating member, such as PTFE (polytetrafluoroethylene) tape, is positioned between the biasing means and the electrode in order to protect the counter electrode from damage. The biasing means can be used to ensure that the electrode is in physical contact with the solid electrolyte. This is particularly important when a non-ionically conducting fluid is used to hydrate the solid electrolyte. In addition, the biasing member ensures sufficient contact between the solid electrolyte and the surface being tested.
The ion exchange membrane can be an ion exchange material selected from perfluorinated sulfonic acid polymers, perfluorinated carboxylic acid polymers, perfluoro bisulfonimide polymers, perfluoro phosphonic acid polymers, perfluorinated tetraalkylammonium salt polymers, carbanion acids, and mixtures thereof. It is preferred that the solid electrolyte has a charge carrier comprising a non-corrosive ion, such as a cation or an anion. A current conductor may be attached to the electrode. The solid electrolyte can have an area of from about 0.1 cm2 to about 10,000 cm2, preferably, from about 1.8 cm2 to about 154 cm2.
In a preferred embodiment, the chamber has an inside surface defining a cavity and a support means positioned within the cavity. The support means defines a bore extending therethrough that is in flow communication with the solid electrolyte. The bore is preferably at least partially filled with fluid, such as water or a liquid electrolyte. The biasing member is positioned between the support means and the electrode and defines a bore therethrough that is in axial alignment with the bore defined by the support means.
The electrode used in the apparatus, may be made from a material selected from platinum, a platinum group metal, gold, a gold group metal, iridium, rhodium, ruthenium, tungsten, titanium, zirconium, stainless steel, carbon, conductive oxides, such as a platinum oxide or oxide forms of a platinum group metal, gold oxides, oxide forms of gold group metals, iridium oxide, rhodium oxide, ruthenium oxide, tungsten oxide, tin oxide, or mixtures thereof. Alternatively, the electrode may be made from a material coated with platinum, a platinum group metal, gold, a gold group metal, iridium, rhodium, ruthenium, tungsten, titanium, zirconium, stainless steel, carbon, conductive oxides, such as a platinum oxide or oxide forms of a platinum group metal, gold oxides, oxide forms of gold group metals, iridium oxide, rhodium oxide, ruthenium oxide, tungsten oxide, tin oxide, or mixtures thereof. The material can be in the form of metal sheets, metal gauze, thin film electrodes, and porous electrodes.
In another embodiment, there is provides a method for detecting corrosion in metallic substrates. The method includes positioning an electronically insulating ionically conducting member in electrochemical communication between a substrate to be inspected and an electrode, applying an AC signal across the electrode and the substrate, and determining an impedance reference value for the substrate. In addition, the impedance value between the substrate and the electrode is also determined. Preferably, the electronically insulating ionically conducting member is a non-corrosive, ion exchange membrane. The surface of the substrate to be tested is preferably moisturized with a non-corrosive liquid such as water, prior to contacting the apparatus with the substrate.
Preferably, a voltage is applied at a selected frequency and an average impedance value is obtained. The impedance reference value is compared with the average impedance value and whether the substrate contains corrosion is determined based on the comparison between the impedance reference value and the average impedance value.
The present invention also provides an apparatus for analyzing corrosion having a solid electrolyte, an electrode in electrochemical communication with the solid electrolyte and a source of a fluid for hydrating the solid electrolyte. The solid electrolyte can be made of an electronically insulating, ionically conducting material, preferably containing a non-corrosive ion. An impedance measuring element may be positioned in electrical contact with the electrode. The source of fluid may be a chamber in fluid communication with the solid electrolyte.
Another embodiment of the present invention provides an apparatus for analyzing corrosion having a sensor having a solid electrolyte, an electrode in electrochemical communication with the solid electrolyte, and a source of a fluid for hydrating the solid electrolyte; and an impedance measuring element in electrical contact with the electrode. The solid electrolyte can be made of an electronically insulating, ionically conducting material, containing a non-corrosive ion, as discussed previously. The impedance measuring element is preferably adapted to generate an AC signal at a selected frequency, measure the perturbed signal, and calculate the corresponding impedance of a substrate being tested. The source of fluid can be a chamber in fluid communication with the solid electrolyte, preferably at least a portion of the chamber is made of an electronically and ionically insulating material.